(1) Field of the Invention
The present invention relates to a process for the preparation of granular .mu.-sulfur having an improved flowability as a vulcanizing agent of rubber composition. According to the present invention, the workability at the rubber compounding operation can be improved and the rubber characteristics of the rubber composition can be improved at the vulcanizing step.
(2) Description of the Related Art
Sulfur has allotropies such as .alpha.-sulfur, .beta.-sulfur and .mu.-sulfur. .mu.-sulfur is a brown liquid sulfur allotropy which is thermodynamically stable at a temperature of at least 119.0.degree. C. physico-chemically. Usually, a commercially available or industrial product is a solid powder prepared by supercooling the liquid .mu.-sulfur by, for example, quenching. Such a commercially available .mu.-sulfur is generally a solid mixture of a supercooled .mu.-sulfur allotropy with a very small amount of .alpha.- to .beta.-sulfur allotropies.
In the present invention, .mu.-sulfur is .mu.-sulfur powder as the above solid powder.
.mu.-sulfur is composed of molecules in which sulfur atoms are bonded like chains extending zigzag. It is a chain-like polymer having a molecular weight of 100,000 to 300,000, which is insoluble in carbon disulfide and rubbers.
In contrast, general sulfur is called .alpha.-sulfur and has an eight-membered molecular structure. It is soluble in carbon disulfide and rubbers, and clearly differs from .mu.-sulfur.
Since .mu.-sulfur has no blooming property at the time of compounding with a rubber, it is widely used in rubber tires.
In general, .mu.-sulfur is composed of fine particles having a particle size of about scores of microns or a smaller particle size, and during the handling, .mu.-sulfur is readily scattered in the form of dusts or in readily statically charged. Therefore, troubles such as fire are readily caused during the handling.
For overcoming this disadvantage, at least one rubber process oil selected from the group consisting of naphthenic oils, aromatic oils and paraffinic oils is incorporated in a powder of .mu.-sulfur and the .mu.-sulfur is used as so-called oil-processed .mu.-sulfur.
This oil-processed .mu.-sulfur is advantageous over unprocessed .mu.-sulfur in that scattering or occurrence of electrostatic troubles is controlled, but the oil-processed .mu.-sulfur is defective in that the flowability is poor and the sulfur adheres to various devices and meters during the handling. For examples, when the oil-processed .mu.-sulfur is measured by a meter and the oil-processed .mu.-sulfur is transferred to the next step, a part of the oil-processed .mu.-sulfur adheres to an inner wall of a vessel to reduce the precision of the measurement, with the result that the composition of rubber is changed and dispersion of the physical properties of the rubber composition is caused.
As the means for improving the flowability of .mu.-sulfur, there is known a method in which oil-processed .mu.-sulfur is granulated by compression molding.
This method, however, is defective in that the .mu.-sulfur agglomerates rigidly and when the .mu.-sulfur is kneaded with rubber, the granular .mu.-sulfur does not collapse to the original powdery state but is left as it is, and therefore, the dispersibility in rubber is reduced and the rubbery characteristics vary widely.
Japanese Unexamined Patent Publication No. 62-246810 teaches a method in which a dry powder of insoluble sulfur and a rubber process oil are mixed at a predetermined mixing ratio, the mixture is granulated and a dry powder of insoluble sulfur is spread on the granulation product. However, this method is defective in that when the mixing ratio of the rubber process oil is low, the powdery insoluble sulfur spread on the surface is scattered and as the amount added of the rubber process oil is increased, an adhesion trouble is caused.